Wireless networks, and particular wireless networks in industrial environments are susceptible to impulsive noise generated by electric equipment. This impulsive equipment noise is commonly characterized by a short duration and high power spike when compared to a desired signal. Thus, the impulsive noise incurs a sudden decrease in instantaneous SNR, which may lead to data packet loss and subsequently poor network performance. Despite the fact that data packet loss can be alleviated by retransmissions, such retransmissions induce delays, which is rather undesirable for wireless industrial networks with stringent delay constraints.
Furthermore, the high-power impulsive noise is particularly detrimental to orthogonal frequency division modulation (OFDM)-based wireless industrial networks. An OFDM block comprises multiple symbols and the whole block has to be jointly demodulated in the receiver to recover the transmitted symbols. As a result, even if short-duration impulsive noise is added to a few symbols, the high-power impulsive noise will be propagated over the entire block after joint demodulation, thereby the entire block, rather than only a few symbols, has to be retransmitted.
FIG. 1 shows the schematic diagram of a conventional OFDM system including a transmitter 110 and a receiver 120. In the transmitter, a signal z(m) is encoded 111, interleaved 112, mapped 113, inverse discrete Fourier transformed 114, parallel-to-serial converted 115, analog-to-digital converted 116, and transmitted on a channel 117 subject to noise n(t).
In the receiver, the received signal r(t) is analog-to-digital converted 121, and blanked 122. Then, the signal is serial-to-parallel converted 123, discrete Fourier transformed 124, de-mapped 125, de-interleaved 126, and decoded 127 to recover {circumflex over (z)}(m).
In the prior art, a noise blanker protects a signal processing circuit from unwanted noise spikes by interrupting the signal path when the noise exceeds a predetermined threshold or reference level, see U.S. Pat. No. 4,479,251 “Noise blanker.”
To cope with the impulsive noise, the blanking 122 is applied. The conventional blanking is characterized by single threshold based on only the amplitude of the received signal y(n) as shown in FIG. 2. In addition, the conventional blanking can only decrease the amplitude to zero.
As shown in FIG. 2, the amplitude of each sample of data symbol y(n) is first compared with a pre-defined threshold T0. If the amplitude is larger than the threshold, then the prior art method blanks the corresponding nth sample of the data symbol y(n)by setting the signal to zero and no other value. Otherwise, the blanker will simply output the sample without change.